150 THE CULTURE OF FARM CROPS. 



silica however are the only parts of the soil which are thus 

 bountifully provided by nature. Every one of the others, 

 even lime, of which 30 tons per acre are contained in many 

 soils and in some there is much more than this is quite 

 rapidly exhausted, so far as the requirements of a full crop 

 are concerned, by a few crops ; for although there may be 

 many tons of lime still remaining in the soil only a small 

 quantity of it is available because it is soluble in water to- 

 a very small extent. The same is true of the potash ; soda ; 

 phosphoric acid; and magnesia; all indispensably necessary 

 to the crops as has been shown above. 



A small quantity of all these elements of plant food is 

 dissolved in the soil by the rain water, aided by the carbon- 

 ic acid which (as has been previously explained) the water 

 holds in solution. The quantity so set free in the soil is the 

 measure of its natural fertility: just as the 7 to 10 pounds 

 of nitrogen which is known to be contributed by the at- 

 mosphere in the form of ammonia and nitric acid, and the 

 few pounds of carbon supplied by the carbonic acid which 

 is also derived from the atmosphere, are the measure of 

 the natural resources of the soil in these respects. This 

 natural fertility is able to support the common spontaneous 

 growth of soil which contains no accumulated stock de- 

 rived from the decay of previous crops. If the soil dug 

 from a deep well is thrown out on the surface and sown 

 with seeds and cultivated, the yield would represent pre- 

 cisely this natural fertility. A very poor growth would be 

 the result. If the precise quantity of all the available ele- 

 ments of plant growth in such a soil could be ascertained 

 and the amount deducted from the known quantities drawn 

 from the cultivated soil by a full crop, we could then cal- 

 culate with reasonable exactness what the soil loses each 

 year, and what the farmer must supply to it to prevent its 

 final exhaustion and preserve it in a fully fertile condition. 



But there are so many accidents of season, and other 

 circumstances, which interfere with the growth of the crops, 

 that the farmer could not safely depend upon such a cal- 

 culation. To be safe, he must leave a very liberal margin 



